This invention relates to a trim/tilt device for a marine propulsion unit and more particularly to an improved hydraulically operated tilt and trim arrangement.
It has been known to mount marine outboard drives such as an outboard motor per se or the outboard drive unit of an inboard/outboard drive on the transom of the hull for movement between a plurality of trim adjusted positions and also for movement to a tilted up out-of-the-water position. Conventionally it has been the practice to employ fluid motors for achieving both the trim and tilt-up operation. A first trim hydraulic motor is mounted on the transom and has a piston rod in abutting relationship with a portion of the outboard drive unit for achieving the trim adjustment. In addition, a tilt cylinder is connected pivotally to the transom and to the marine outboard drive and when actuated causes tilt-up operation.
It is generally the practice to provide a relatively higher power but lower speed hydraulic motor for the trim adjustment than for the tilt movement. The reason for this is the trim adjustment is normally made under running with the outboard drive generating significant driving thrust against which the trim cylinder must operate. The tilt cylinder, on the other hand, only need lift the weight of the outboard drive above the water. It has generally been the practice to incorporate a hydraulic circuit that will accomplish first the trim adjustment and then when the trim cylinders are at the end of their stroke, a tilt movement. Of course, this type of arrangement provides a relatively complicated structure and incorporates at least two fluid motors each of which has a piston rod that is exposed at the rear of the transom to the marine environment, which can obviously cause corrosion problems.
In order to simplify the construction and operation, it has been proposed to employ an arrangement wherein the trim and tilt cylinders are formed within a common concentric unit. This unit includes a cylinder housing that is affixed to the transom and which defines an internal chamber that is divided into two cylinder portions by a trim piston. The trim piston itself defines an internal chamber in which a tilt piston is slidably supported to define two trim piston chamber portions. The chamber portions of the trim piston and cylinder are communicated with each other at their respective ends. As a result, a hydraulic circuit can be employed which will pressurize both the tilt and trim pistons until the trim piston is at the end of its stroke and then tilt-up operation will be achieved by continued movement of the tilt piston within the trim piston.
It is also the practice with this type of arrangement to incorporate a shock absorbing structure, generally in the tilt piston, so as to permit the outboard drive to pop up when an underwater obstacle is struck. However, when this is done with the single concentric assembly of the type described in the preceding paragraph, the outboard drive can also pop up when operating under reverse thrust conditions.
With a conventional system employing separate tilt and trim cylinders, the shock absorbing structure within the trim cylinder will resist popping up under reverse operation. However, with the concentric type of device previously described, when operating in reverse mode, the tilt piston tends to be drawn upwardly within the trim piston. This causes a transfer of fluid from the tilt piston chamber portion to the corresponding cylinder chamber portion through the open passageway that exists between these chamber portions. This causes the tilt piston to be driven downwardly so as to accommodate this fluid flow and the outboard drive can easily move up under reverse thrust, an obviously undesirable situation.
It is, therefore, a principal object of this invention to provide an improved concentric tilt/trim cylinder arrangement for a marine outboard drive that can resist reverse thrust operation while at the same time permit popping up when an underwater obstacle is struck.
It is a further object of this invention to provide an improved concentric tilt and trim cylinder arrangement for a marine outboard drive that does not have the deficiencies of prior art type constructions.
As has been previously noted, the prior art type of concentric tilt and trim cylinders have operated in such a way that the trim piston moves through its full upstroke simultaneously with the tilt piston and when the trim piston reaches the end of its stroke, the tilt piston will continue to move upwardly but at a higher speed and with a lower force. Reverse operation occurs in the opposite direction. That is, when tilting down the tilt piston is forced downwardly along with the trim piston until the tilt piston reaches the bottom of its stroke and then the trim piston will continue to move through the remainder of its stroke. However, when a down force is exerted and the device is not in its fully tilted-up position, another problem can occur with the prior art type of devices. That is, a fluid pressure is exerted first in the chamber portion of the cylinder above the trim piston and as the trim piston is forced downwardly, the pressure in the opposite portion of the cylinder chamber will rise. The downwardly action hydraulic force must pass to the tilt piston through a restricted opening and the downward pressure on the tilt piston will not rise as rapidly as it is on the trim piston. However, the fluid pressurized in the under side of the assembly will act upon the tilt piston and at times this pressure can be higher than the downward pressure on the tilt piston. Thus even though the operator desires to trim the outboard drive downwardly, it will initially rise. This is obviously a disadvantage.
It is, therefore, a still further object of this invention to provide an improved hydraulic circuitry for actuating a concentric tilt and trim cylinder assembly.
It is a further object of this invention to provide a tilt and trim cylinder assembly of the concentric type wherein it will be ensured that trim down operation is always accomplished the instant it is called for.